crypto: x86/poly1305 - add back a needed assignment

One of the assignments that was removed by commit 4a0c1de6 ("crypto:
x86/poly1305 - Remove assignments with no effect") is actually needed,
since it affects the return value.

This fixes the following crypto self-test failure:

    alg: shash: poly1305-simd test failed (wrong result) on test vector 2, cfg="init+update+final aligned buffer"

Fixes: 4a0c1de6 ("crypto: x86/poly1305 - Remove assignments with no effect")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>